This invention relates to thermal printing apparatus and in particular to the construction of printing heads for use in thermal printing apparatus.
Known thermal printing apparatus operates by selectively heating areas of a thermal transfer ribbon, comprising a substrate film carrying a layer of ink, to effect transfer of ink from the ribbon onto the surface of an item on which printing is desired. Generally a thermal printing head is utilised which has a row of selectively heatable print elements and these print elements are heated in synchronism with the passage of the surface, on which printing is to be effected, past the print elements. A known construction of thermal printing head comprises a substrate acting as a heat sink on which a strip of electrically resistive material is deposited. Electrically conductive tracks are deposited to provide a plurality of electrical connections at positions spaced along the length of the resistive strip, layers of electrically insulating material being deposited to electrically insulate the conductive tracks from one another. By causing electrical current to flow from one selected connection to another selected connection through that portion of resistive element extending between those selected connections, that portion of resistive element is heated. The resistive element is disposed to extend across the direction of travel of the item to be printed and accordingly any desired pattern may be printed by repeated selective electrical energisation of selected portions of the resistive element as the thermal transfer ribbon and the surface of the item to be printed are moved past the resistive element. The substrate of the printing head acts as a heat sink to carry away the heat generated in energisation of the resistive element in order to prevent the energisation of the portions of the element having a persistent effect on the transfer ribbon.
Known constructions of thermal print head tend to be relatively slow in operation compared with other forms of selective printing heads. While it has been possible to print in more than one colour during a printing operation by the use of multi-colour ribbons, multi-colour ribbons are expensive, complex and inflexible. It is desirable to increase the speed at which printing can be effected and to be able to select the colour of printing as desired during a printing operation.